nir/lower_variables: Improve documentation
nir/lower_variables: Improve documentation

Additional description was added to a variety of places.  Also, we no
longer use the term "leaf" to describe fully-qualified direct derefs.
Instead, we simply use the term "direct" or spell it out completely.

Reviewed-by: Connor Abbott <71178acffcc112b21e5858656e5751f5e4aa9364@gmail.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler